The Virtual District Line

built to a standard, not a price


Visitors to London acquainted with Network Rail signalling practice doubtless find certain aspects of London Underground's signalling a touch confusing. Signals momentarily showing both red and green aspects or what appears to be a four aspect signal showing green and yellow are not what people brought up on the mainline are used to.

Automatic Signalling

The layout of signals is similar to the absolute block system. The station, or junction, is the block post. At the front end of the platform is the Starter. If there is a diverging junction just beyond the platform there may be Advance Starters on each branch.

The signals approaching the station are the Home signals, of which there may be a number. At King's Cross on the eastbound Piccadilly Line there used to be five, the first two were speed controlled. Under speed controlled signalling if the driver had not reduced speed to a predetermined level the signal will remain at danger and if passed the train will be tripped resulting in an emergency brake application.

As the train in the platform departs the home signals will clear in succession and the following train can enter. The whole system is designed to reduce headways and keep trains moving.

Depending on the distance to the next station there may be intermediate running signals to reduce the length of the section.

Layout of Signals.png

Signal Layout Diagram

Lineside Signals

Running signals are the usual red for stop and green for proceed with yellow replacing the red for repeaters.

As on Network Rail, Automatic Signals are controlled by the passage of trains through track circuits. They normally show a green aspect until a train passes or a fault occurs.

What on NR is called a "controlled signal" is on LU a Semi-Automatic Signal. The normal aspect is danger and it only clears when the signalman, or a programme machine or computer, operates the control. As the train enters the section it automatically returns to red. "Autos" are used on plain track and "Semis" at junctions.

Different sizes of signal are used in tunnels and on open sections. In tunnels the lens is 4½ inches diameter and designed to spread the light. 8 inch lenses are used on the surface with a narrow beam for long distance sighting. As an aid for very close range on surface section signals, a supplementary lens, (known as a "pig's ear") is fitted at the side of the main lens.

Semi-Automatic Repeater signals showing Clear and Caution

Semi-Automatic signals showing Proceed and Stop.

The apparent green and yellow mentioned earlier is in reality two separate signals. The green is the proceed aspect of a stop signal while the yellow is the caution aspect of a repeater (not a distant). When the stop signal shows danger both aspects of the repeater are extinguished.

Stop signal at Proceed and Repeater at Caution

Stop signal at Proceed and Repeater at Clear

Stop signal at Danger and Repeater Extinguished

Repeaters are used where sighting distances are restricted and each one is for a designated signal. Sometimes a repeater is for more than one stop signal, but not always the next.

To improve sighting in bad visibility additional repeaters are used. These are a miniature yellow/green signal with a white surround marked "Fog Repeater". In multiple aspect area the yellow is replaced by a lunar white lamp with an "F" on the lens.

Identifying Signals

Two types of signal number plates are used. Black on white for stop signals and black on yellow for repeaters. Semi-automatic signal numbers correspond with a lever in the signal cabin or interlocking machine room (relay room) and are prefixed by the cabin code. Automatic signals are sequentially numbered and prefixed by the letter "A".

Station Signals

At stations where the station starting signal is not easily visible from the rear of the train Platform Repeaters are installed to help station staff. They show yellow and green aspects as well as Junction Route Indicators and Shunt signals.

A Fog Repeater
showing Caution.

Platform Repeater showing Clear with a repeater for the Junction Route Indicator (see below) showing the diverging route

Platform Repeater showing the Shunt Signal at Clear

Preventing signal overruns

Integral parts of the signalling system are the Trainstops and Tripcocks.

The Trainstop is securely attached to the right hand ends of the sleepers alongside the signal. Various electrical proving circuits pass through so that if there is a fault on this or the next trainstop ahead then it will remain up and the red aspect will stay alight giving a "dual aspect" if the section ahead is clear.

The tripcock is part of the train equipment. Fitted to the right hand leading positive shoebeam it is an emergency brake valve. In its normal position, pointing straight down, the valve is closed and the brake can be released by the driver. If the driver should overrun a signal at danger the arm strikes the trainstop. It is knocked back, opening the valve allowing the air in the brake system to escape to atmosphere and applying the brake. The air escaping opens a pressure switch which breaks the control circuit and cuts power to the motors.

To reset it, the driver must assist the train to stop and then pull a cord on the front exterior of the train.

Trainstop lowered, signal Clear.

Trainstop raised, signal at Danger.

Direction of Travel

Front Right Hand Shoebeam


Running Rail

Box, fixed to sleeper ends,

containing Air Valve,

Spring and Detection eqt.

Trainstop arm which rises when the signal is at Danger and falls when the signal is showing Green.

The train has over run the signal showing danger, the tripcock arm has struck the trainstop arm causing the tripcock valve to open, apply the brakes and bring the train to a stand.

The train has passed the signal showing Proceed, the trainstop arm was lowered and so the tripcock arm passes safely over it.

In order to be certain that the trip arm has not been damaged during a journey, testers are placed at various locations along each line. To check horizontal alignment the arm passes between two upright posts and to pass the vertical test it must depress a spring loaded ramp. The driver's proof that all is in order is that the train is not tripped and that a dedicated lamp adjacent to the platform starter goes out. This check is carried out at least once each journey.

Tripcock Tester

Signals sometimes show a momentary dual red/green aspect. This is caused by the associated trainstop arm being slow to drop when the signal goes to green. The section ahead being clear allows the proceed aspect to illuminate but the red stays lit until the trainstop is proved to be fully down. The trainstop arm is held up by springs and forced down by air pressure. Some are a bit stiff and slow to move. A cause of trainstops failing to go down fully is rubbish, such as drink cans, becoming trapped underneath. If the trainstop fails to go down completely then a dual aspect will continue to show.

Supplementary indicators

Route indicators are take the form of line of white lights on top of junction signals. The correct name is “Junction Route Indicator” but are more often referred to as known as “Harbour Lights”.

Junction signal cleared for the diverging route

Moving to and from sidings and depots

To indicate to a driver that he is about to make a low speed movement into or out of a siding or yard a different type of signal is used.

Shunting and subsidiary signals are white discs 15" inches in diameter and externally illuminated. Shunt signals have a horizontal red band rotate 45 degrees anti-clockwise to show "proceed".

For shunting moves to two or more routes a matrix of individual lamps called a Theatre Route Indicator is used. When a move is set up "1" is used for the leftmost route, "2" for the second from left and so on.

Shunt signal at Proceed with Theatre Route Indicator showing that the route has been set for the second siding.

Allowing two trains in one platform

A legacy of past practice can still be seen in one or two locations. Subsidiary signals are no longer in use but they are again white 15” discs but with two red lines across the middle and carry either "W" for a Warning Signal or "C" for a Calling On Signal. Subsidiaries were used when a second train was to enter an already occupied platform allowing two trains to couple.

Calling on signal at Danger

These signals became redundant on the District Line in 1971 when it was decided that all trains would be formed of seven cars and coupling of trains in service was discontinued.

Signals specific to LU

There are signals peculiar to LU. Two of these are "X" signals and "Draw-up" signals.

The first of these is sited before special items such as floodgates and as the first signal after a Surface Stock Detector. This is a device sited on the approach to a tube stock loading gauge tunnel where surface stock might inadvertently gain access, such as the Piccadilly Line tunnel mouth at Baron's Court. They consist of a gantry with three "U" shaped mercury filled glass tubes suspended sited on the approach to a semi-automatic signal which is normally showing Stop. If a surface stock train tries to pass the tubes will be broken and the signal will remain at danger.

Draw-up signals are miniature speed controlled three-aspect signals sited partway along platforms where there is a converging junction immediately ahead of the platform starter. When the route is set for a train to join the track in question an approaching train has to reduce speed to a predetermined level when the draw-up signal will clear to a yellow. The incoming train can then continue to the stopping mark.

Draw Up signal at Danger. Train speed being tested and must be prepared to stop.

Draw Up signal at Caution. Station Starting Signal at Danger.

Draw Up signal at Clear. Station Starting Signal at Clear.

Speed control signals and equipment

Following the Moorgate incident in 1975 special measures were introduced at terminal stations, Firstly trains have their speed reduced by speed control signals on the approach. The traction current supply is then restricted to prevent the driver applying power to the motors. When entry to the platform is on an up grade limited current is available to overcome its effects.

Sited along the platform are "Policemen". These are trainstops without a signal and unless the train continues to reduce speed will remain raised and operate the trainstop which will apply the emergency brake.

Traction current

Traction current rail gaps are marked by special signals which only illuminate when the current is off in the traction current section ahead. "Rail Gap Indicators” consist of three red lights in a white triangular plate marked as such.

Their repeaters are similar but both lights and plates are yellow.

Rail Gap Indicator showing that the Traction Current is off in the section ahead.

Signalling on both the Victoria and Central Lines is different as they are equipped for Automatic Train Operation.

Chiltern Railway class 165s permitted to operate over LU metals and those mainline trains operating in single track tube tunnels are also fitted with tripcocks.

Text & Images © Roger Viggers

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Tripcock arm